I'm getting a Gatewood Cape and I'm considering replacing the guylines with the BPL kit primarily because of the cam-lock tensioners (silnylon does stretch, and the tensioners would make it easier to snug things back up in the middle of a rain storm).

However, I've never heard of anyone's real-life experience with this product.

I've used them in mountaineering applications where wind gusts got to well over 60mph. In cases where I could only crawl and prayed for my life, the guylines held well. Even though I SHOULD use supplied guylines when doing reviews of bomber tents, I will sometimes trade them out for these if I'm worried. :-)